It has been suggested that the proliferation of T lymphocytes cultured with autologous non-T cells is an MHC-restricted response to xenoantigens. We have found that the presence of antigens such as sheep erythrocytes or fetal calf serum may augment the autologous MLR but are not required for this reaction. The enhancement of the autologous MLR in some donors probably reflects their sensitization to the xenoantigens present in culture. T cells activated in the autologous mixed lymphocyte reaction (AMLR) lyse a wide variety of target cells including autologous and allogeneic non-T cells, mitogen blasts and natural killer (NK) susceptible cell lines devoid of HLA determinants. Autoactivation is required to generate these killer cells as neither fresh T cells nor T cells cultured alone for 7 days are capable of lysing any target cells. Percoll gradient fractionation of cells from a 7-day AMLR demonstrated that the killer cells to both blast cells and K562 targets were present in the low density fraction. Lysis of these autoactivated populations with OKT8 and complement (C') substantially diminished ability of the remaining to lyse mitogen blast target cells while slightly augmenting the lytic activity to K562. Conversely, lysis with OKM1 and C' reduced the cytotoxic activity toward K562 without diminishing the killing of the mitogen blasts. Moreover, the addition of increasing numbers of cold, unlabeled K562 effector cells and labeled mitogen blasts did not inhibit the lysis of the blast targets. Conversely, unlabeled mitogen blasts did not inhibit the lysis of K562 targets. Data suggest the two distinct populations of killer cells are activated during the AMLR: one which is OKT8+ and lyses NKresistant mitogen blasts and a second which is OKM1+ and lyses the NK-susceptible myeloid line K562. The goals for the coming year are: (a)\establishment of lines and clones of auto-\and alloreactive T lymphocytes; and (b)\investigation of the fine specificity of the receptor on auto-reactive T cells.